Magnetic leakage field flaw detector utilizing partially overlapping hall probes

ABSTRACT

Method and apparatus for detecting flaws in non-destructive testing of material having a longitudinal extent, e.g., in welds of butt-welded pipe, through the use of a flux energizer having magnetic poles adapted to impart flux to said material in a path transverse to said extent, and a pair of flux sensors. The flux sensors comprise a pair of overlaying Hall generator devices arranged so that a line extending between the voltage lead connection points of one device is parallel to a corresponding line of the other device and to a line transverse to the ambient flux path.

Umted States Patent 1191 1111 3,710,236 Halsey et al. [4 11 Jan. 9, 197354 MAGNETIC LEAKAGE FIELD FLAW 3,609,530 9/1971 Johnson ..324 37DETECTOR UTILIZING PARTIALLY 3,125,719 3/1964 Seabury, Jr ..324/37OVERLAPPING HALL PROBES 3,619,769 ll/197l Kusenberger 324/37 [76]Inventors: Homer P. Halsey; John F. Halase, O GN P TENTS 0R APPLICATIONS111, both of 0/0 Youngstown Sheet 950 696 2 1964 G tB 324 3 and TubeCompany PO. Box 900, rea r1 a1n 7 Youngstown Ohm 44501 PrimaryExaminer-Robert J. Corcoran [22] Filed: Nov. 30, 1970 A n y-J hn SteImahMethod and apparatus for detecting flaws in non- UIS n I s I s a [51]III!- Cl. ..G01g 33/12 extent, 6%" in welds of butt welded pipe throughthe [58] held of Search ..324/37, 40, 45; 307/309; use of a fluxenergizer having magnetic poles adapted 332/32 H to impart flux to saidmaterial in a path transverse to said extent, and a pair of fluxsensors. The flux sensors [56] References cued comprise a pair ofoverlaying Hall generator devices UNITED STATES PATENTS arranged so thata l ne extendmg between the voltage lead connectlon pomts of one dev1ce1s parallel to a 2,351,944 6/1944 Engler ..324/37 corresponding line ofthe other device and to a line 3,579,099 5/1971 Kanbayashi ..324/37transverse to the ambient flux path 1,998,952 4/1935 Edgar et al...324/37 2,855,549 10/1958 Kuhrt et a1. ..324/45 6 Claims, 8 DrawingFigures PATENTEDJAN 9 I975 3.710.236

sum 2 BF 2 INVENTOR5= HOMER P. HALSEY JOHN F. HALASE III by W TH IRATTORNEY MAGNETIC LEAKAGE FIELD FLAW DETECTOR UTILIZING PARTIALLYOVERLAPPING HALL PROBES BACKGROUND OF THE INVENTION This inventionrelates generally to non-destructive detection and testing for flawsthrough the use of a magnetic field, and particularly to the interceptorsensing and depicting of minute flaws, such as hairline cracks, throughthe use of a magnetic flux leakage detector system.

Some of the features of this invention are particularly adapted for thedetection of end-cracks which develop or occur in the welds of buttweldpipe. How ever, these features will also prove useful and advantageousin conjunction with the detection of cracks throughout the extent of alongitudinal weld and/or of other flaws which have a longitudinalextent.

In the non-destructive detection of flaws in products formed of orcontaining magnetic material, it is desirable to provide a system whichnot only performs with a high degree of accuracy but which will alsodiscriminate certain types of flaws from other flaws, particularly thoseother flaws which may not be objectionable. This desideratum isparticularly important in the end-crack detection of buttweld pipewelds.

Magnetic flux leakage detectors are known in the art. One of the methodsfor detecting flaws by the flux leakage process consists of imposing amagnetic field in the test specimen and sensing the distortion of theflux pattern which occurs adjacent any defect in the specimen. Thedistorted flux is sensed by an electromagnetic search coil whichgenerates a signal. How ever, such search coils are radial velocitydependent and in such arrangements the output signal isproportional tothe rate of change of flux per time unit. Also, such search coils areunwieldy and inherently limited because of their size and configuration,particularly in connection with large diameter tests specimens. Further,another disadvantage of such method. and ap paratus is the inability todiscriminate cracks from other types of flaws or irregularities.

Eddy current type method and apparatus have also been employed fordetecting flaws. However, they are deficient in some of the samerespects as are the sensing coils, e.g., inabilityto discriminate cracksfrom other flaws. Also, such method and apparatus are very sensitive tosurface irregularities, to out-of-roundness of the test specimen (whentesting cylindrical specimens), and are extremely affected by specimenedge effects, to the point of being ineffective in sensing flawsoccurring at the edges of specimens.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of this inventionto provide method and apparatus for improving the sensing andmeasurement of magnetic fields in the detection of flaws in magnetizedmaterials.

It is another object of this invention to provide method and apparatusfor preferentially sensing and depicting flaws in magnetizablematerials, which flaws have a longitudinal extent.

It is a further object to provide method and ap' paratus for sensingmagnetic flux and translating the sensed flux, to an accentuated output.

It is a still further object to provide method and apparatus which isparticularly adapted for sensing and depicting end-cracks in weldedpipe.

Briefly, the objects are attained by providing means for magnetizing thetest specimen and sensing means for sensing the flux leakage whichoccurs at a flaw in the specimen, which sensing means is characterizedby a pair of spaced apart effective sensing regions which are parallelto each other and about a line transverse to the ambient flux pathbetween the poles of the magnetizing means. The most effective sensingregions are positioned on opposing sides of a flaw at the instant whenthe flaw is relatively moved across one of the sensors. Preferably, themost effective sensing regions are of longitudinal extent and are spacedapart a distance which generally corresponds to the width of weld cracksor other flaws which are generally expected to be encountered.Accordingly, the orientation is such that a plane traversing thelongitudinal extents of said regions is not normal to the planes of saidregions. Also, preferably, the output generated by one of the sensingdevices is subtracted from the output of the other. Thus the outputsgenerated by ambient flux patterns tend to cancel each other. However,the sensors are adapted to be positioned about a flaw, so that theoutputs attributable to the flux pattern of the flaw are opposite inpolarity. Accordingly, when the fllaw moves across the sensors or whenthe sensing regions are moved across a crack, a comparatively sharpimpulse, in comparison to other flaw signals, is generated. Means arealso provided for selectively filtering, and for amplifying thesummarized signal. The signal is then rectified and compared with apreselected set amplitude level. When the amplitude of the summarizedsignal is greater than the preselected level, an audio and/or visualflaw indicating signal is preferably generated to indicate the rejectstatus of the tested specimen.

DESCRIPTION OF THE DRAWING The invention will be more fully understoodand further objects and advantages thereof will become apparent whenreference is made to the following detailed description and to theaccompanying drawing, in which:

FIG. 1 is a block diagram of an embodiment of this invention,particularly illustrating a preferred embodiment of the electriccircuitry for translating sensed magnetic flux to a flaw indicia signal;

FIG. 2 is a schematic diagram illustrating in more detail the embodimentof FIG. 1; and

FIG. 3 is a three-dimensioned schematic representation of magnetizingand flux probing devices in accordance with this invention;

FIG. 4 is a three-dimensional view of a preferred type of unit inaccordance with this invention embodying termediate the flux sensingregions and an attendant typical flux pattern;

FIG. 8 is an end elevational view illustrating an arrangement wherebythe magnetizing/sensing unit and a pipe test specimen are relativelymovable, one to the other, to facilitate scanning of a longitudinal weldof a pipe.

DETAILED DESCRIPTION In a representative embodiment of this invention asblock diagram illustrated in FIG. 1, the apparatus may be considered tobe generally comprised of three sections, generally designated as: amagnetic flux energizer 12, an sensing device 14', and'an outputtranslating section 16, for translating the sensed flux to a flawsignal. The energizer section 12 and sensing unit 14 may be convenientlyincorporated in a single hand-holdable unit as shown in FIG. 4 andgenerally designated and referred to as energizer sensing unit 10.

The magnetic flux energizer section 14 preferably includes anelectromagnet 20 (FIG. 3) having core pole sections N & S adapted to bepositioned about the circumference of a test workpiece, such as pipe 18.The end faces 22 and 24 of poles N & S, respectively, are preferablypositioned as closely as feasibly possible to the circumferentialsurface of the workpiece 18, without touching and with reasonableallowance for out-of-roundness. When testing for end-crack flaws in theend weld portion of a pipe, for which testing this invention isparticularly adapted, the magnetizer/sensing unit may be convenientlyprovided with mandrel 26 for centering the pipe 18 in respect to thepoles N & S. Separate mandrels 26 may be provided for each size range(inside diameter) of pipe. The pole pieces N & S may also be detachablymounted to facilitate positioning and selection of pole piece sectionswith end face curves adapted to the outside diameter of the pipe to betested. Alternatively, the mandrel may be of the expansible type andthus accommodate several sizes of pipe. However, in large scalecommercial operations it is preferable to provide a separate unit 10 toaccommodate each size range of pipe.

The unit 10 is also preferably provided with a mounting assembly 28 forsuitably positioning the effective sensing regions of sensing device 14in a plane corresponding to the plane through which the line desired tobe scanned extends (when viewed from a longitudinal side of the pipe),i.e., with pipe having unbeveled ends, the line would generally be in ahorizontal plane when the weld is top-side of the pipe, but with a pipehaving a beveled end to be scanned, the line would be considered to bein a plane tilted from the horizontal to a degree corresponding to thedegree of bevel. In each case, however, the effective sensing regionsextend in a direction transverse to the ambient flux field between polesN & S.

As indicated above, the energizer/probing unit 10includes a sensingdevice 14, which in a preferred form includes two Hall devices,designated for convenience as 30R and 30L, the combination forming adifferential sensing device. Also for convenience, the correspondingparts of the Hall devices bear corresponding numerals but have lettersuffixes corresponding to the respective Hall device, and in thedescription the letter SUfflX is omitted when the description isappropriate to both Hall devices. Each of the Hall devices includes aconductor, or semi-conductor, element 31 which includes the preferredsensing region 32 for sensing any magnetic flux lines fringing thesurface of the mag netized test workpiece 18. As may be seen in FIGS. 3,

6, and 7, at least a portion of the Hall devices overlay each other, andas may be seen in FIG. 5, the planes of conductors 31 R and 31 L overlayeach other, with the planes being defined by the width and length of theconductors. The overlaying is in a direction normal to the position ofthe surface to be tested, as opposed to being spaced along the surface.

I It is understood that heretofore the entire section of the conductoror semi-conductor material in a Hall device has been considered as beingthe active or sensing area. We have discovered, however, that the regionbordering a straight line connecting the voltage lead connection pointsV, (voltage lead connecting line) is more sensitive to flux pick-up thanother areas of said material. In a preferred embodiment of thisinvention, the Hall devices are positioned so that their respectivevoltage lead connecting lines are parallel to each other and transverseto the ambient flux path between the poles and the flux path through thepipe. Thus these regions are designated as the most effective sensingregions" of longitudinal extent. Unexpectedly, it was discovered thatthe sensitivity of the sensing devices is increased in the order of 10to 20 times when they are arranged with their voltage lead connectinglines parallel to each other and transverse to the ambient flux path ascompared with an arrangement wherein the current lead connecting linesare parallel to each other and transverse to the ambient flux path.Consequently, such an arrangement renders the method and apparatus ofthis invention particularly adapted for detecting minute cracks whichotherwise might be lost in the noise."

The two Hall devices 30R and 30L are arranged so that the longitudinalextent 33 of their respective most effective sensing regions 32R and 32Lare generally parallel to but spaced from each other. In other words,the orientation may be described as being such that a transverse plane Pextending through the longitudinal extent of the straight lineconnecting the voltage lead connection points V, of device 30R and thecorresponding Iine of device 30L, is not normal to the plane of eitherdevice. When the energizer/sensing unit 10 is used for the detection ofend-cracks in welded pipe, the spacing between the most effectivesensing regions is in the general order of one-sixteenth inch. In a unit10 that has been built and tried, the Hall devices were arranged with aportion of the plastic matrix overlaying the matrix of the other.However, it is envisioned that the two conductor portions 31 could beincorporated in a single matrix.

The conductors 31R and 31L, with their respective sensing regions 32Rand 32L, are preferably fixedly positioned, by means of sensing mountingassembly 28,

about a line corresponding to the centerline between the poles N & S ofenergizer 12. The most effective sensing regions 32 are adapted to bepositioned about the test piece 18 with their longitudinal extentsgenerally parallel to the longitudinal extent of the test piece 18.Hence, as will be explained and will become apparent hereinafter, themethod and apparatus of this invention is particularly adapted to testfor weld cracks extending longitudinally in welds of buttweld pipe.

As previously indicated, the two Hall devices 30 may be considered tocomprise a differential sensing. The conductors 31R and 31L areelectrically connected with a summarizer in the form of differentialgaussmeter 40 so that the output voltages developed by Hall devices 30Rand 30L are subtracted electronically. The net effect under ambientconditions (in the presence of no flaws) is for the outputs to canceleach other. Thus noise and other ambient flux conditions common to bothconductors 31R and 31L are cancelled. This effect is schematicallyillustrated in FIG. 6, where there is shown a typical flux patternproduced by a test specimen section having no flaws in the scanning areaof the conductors 31R and 31L. The output voltage e that is developed bydifferential gaussmeter 40 is proportional to the vector difference ofthe flux fields B and. B sensed at regions 32,, and 32,, respectivelyand may be expressed as e=K(B -B where K is a constant. Under ambient"conditions, the magnitudes of B and B outputs are substantially equalsince they are generated by the same flux pattern. Therefore, if outputB is subtracted from output B, the differential gaussmeter 40 willproduce a weak or no output.

On the other hand, if a crack or other flaw is present on the surface ofthe test workpiece l8 and is selectively positioned (the workpiece orthe sensing regions being rotatable in respect to each other) so thatthe effective sensing regions 32R and 32L oppose each other fromopposite sides of a crack, a different flux pattern will be sensed.Under such condition, the magnetic flux pattern will fringe and exhibitopposing poles n, and s, at the opposing sides of the crack, asschematically illustrated in FIG. 7. When the sensors, in the form ofsensing regions 32R and 32L are thus positioned, sensor 32L senses theambient flux, which may be designated as B and the flux of the fringingmagnetic lines diverted by the crack, and which may be designated as BCorrespondingly, sensor 32R senses ambient flux B and the flux generatedby fringing magnetic lines, designated as B When the sensors 32R and 32Lare substantially equally spaced about the crack, the fluxes B and B aresubstantially equal; however, they will generate outputs which areopposite in polarity, since the fringing magnetic lines move throughsensor 32L in a first direction, which for purposes of convenience ofillustration may be designated as up or with a positive polarity, andback through sensor 32R in an opposite direction, which may bedesignated as down or with a negative polarity. Now the flux fields asintercepted or sensed by sensor 32L and 32R may be expressed,respectively as:

BL u u; and a BRA ar The voltage e that is then developed may beexpressed as:

e= 14 u) (BRA ar)- Where I BLA BRA and u It will thus be apparent, thatmethod and apparatus are provided for substantially ac centuating theoutput generated by magnetic flux lines sensed adjacent a flaw, such asa longitudinal hairline crack, in contrast to method and apparatus whichaccentuate the flux pattern of a flawless test specimen section. Weselectively tune" the sensors to a crack flux pattern by selectivelypositioning the sensors 32 apart a distance generally corresponding tothe separation of the n, and s, poles produced by the crack. Theparallel positioning of the longitudinal extents of the sensors 32 withthe crack also contributes to increased sensitivity by the apparatusbecause a maximum number of flux lines, which extend generally normal tothe test section surface and across the gap, are sensed. in contrast,those flaws which are not so oriented will produce a weaker outputbecause fewer flux lines are sensed by the sensors 32. Furthermore, thesubtraction of one sensor output from the other sensor output results inan effective flaw signal which is virtually free of ambient noise andother ambient flux conditions which are sensed and generallyincorporated in the final signal output of single probe devices. Thedevelopment of two output voltages which are attributable to a flaw andwhich are substantially equal in magnitude but opposite in polaritycontributes to the accentuation of a flaw signal.

Additional electronic circuitry is also provided to further selectivelyamplify and refine the flaw output voltage and will now be described.The pulse output generated by the differential gaussmeter 40 is noisefiltered and amplified by means of a high pass filter and amplifierstage 42 formed by means known in the art (FIG. 2). The amplified outputis then passed through a diode clipper circuit 44, where only apreselected portion, above or below a preselected magnitude level, ispermitted to pass. The "passed" output is then rectified by rectifiercircuit 46 so that it may be suitably compared with a preselected levelpreset in a comparator 48. If the rectified passed output deviates fromthe preset level in the comparator 48, an output is transmitted to amonostable multivibrator 49 to relay driver 50, to drive relay 52. Relay52 in turn energizes flaw indicator 54. The flaw indicia is preferablyin the form of a signal light but may take other form, such as anaudible signal.

The method and apparatus of this invention is particularly adapted fortesting of flaws which tend to occur at the ends of test work specimensbecause of the cancellation of noise" signals which otherwise areusually attendant because of edge effects upon the magnetic fluxpatterns. However, the concepts of: providing longitudinally andparallel extending flux sensors for traversing opposing sides of areaswhere longitudinally extending flaws are likely to occur; of translatingthe outputs of the sensors to cancel ambient condition signals; and ofaccentuating the output attributable to a sense in change of fluxpattern may be utilizedin other applications, such as in the scanning ofthe entire length of the weld in buttweld pipe.

In FIG. 8 there is schematically illustrated an arrangement whereby theentire weld length of the test specimen may be scanned. Theenergizer/sensing unit 10 is suitably supported by support means 60 sothat core poles N & S and sensing device section 14 are radiallyadjacent to the workpiece 18. Support rails 62 may be provided tosupport workpiece 18. One or both of the support means 60 and supportrails 62 should be movable in a manner to facilitate longitudinal probescanning of the pipe length. The pipe should also be rotated as it isremoved along a longitudinal path so that any point on the pipe willgenerate a helical line as it advances, a sufficient length of scanningarea, whether defined by one or more sensors, should be provided tointercept the length of weld which will advance longitudinally duringone revolution of the pipe.

Referring now to FIG. 2 for a more detailed description of theelectrical circuitry of this invention:

Resistors R1 and R2 determine the gain of the opera tional amplifier A1.Resistors R28, R31, and R47 are used to stabilize and balance amplifierAl. Capacitor Cl attenuates low frequency signals, whereas capacitor C2lowers the amplifier gain for high frequency signals. When used inconjunction with each other, the above circuit elements serve as anamplifier-bandpass filter combination.

Diodes D1, D2, D5, and D6 create a dead zone in the circuitsssensitivity; signal levels between approximately 1.4 and 1.4 volts areinhibited by the diodes. This range of signal levels is characteristicof noise levels observed at the output of the first operationalamplifier.

Resistors R4, R5, R6, R9, and R10, and diodes D7 and D8, in accord withoperational amplifier A2, constitute a rectifier circuit. This portionof the circuitry guarantees that the signal applied to the next portionof the circuit will be of a positive value. This single polarity valuemakes for easy comparison. Resistors R7, R29, R30, and R48 are used tostabilize and balance amplifier A2.

Resistors R8, R11, R44, R45, and R46 provide an adjustable negativepolarity voltage which is indicated on meter M1. This negative voltageserves as a sen.- sitivity adjustment when used in conjunction withoperational amplifier A3.

Resistors R13 and R33, and Zener diode Z1 cause A3 to respond asfollows: If the negative polarity voltage is larger than or equal to thepositive signal from A2, the output of A3 is very near zero volts. Ifthe negative voltage is smaller than the positive signal, the output ofA3 goes negative to a value equal to the Zener voltage. This negativeoutput is used to trigger the flaw indication circuitry. Therefore, byvarying the magnitude of the negative voltage applied to the input ofA3, the positive signal level required to make the A3 output negative isadjustable. Resistors R12, Rl4, and R49 serve to stabilize and balanceA3.

Capacitor C3, resistor R16, and diode D10 serve to condition thenegative output of A3 to a signal required to trigger the followingcircuitry.

Resistors R15, R17, R18, R19, R20 and R22, capacitor C and transistorsQ1 and 02 comprises a monostable multivibrator circuit. When triggeredby the negative output of A3, this portion of the circuitry generates apulse approximately one second in duration.

LII

Resistor R21, capacitor C6, diode D9, transistor Q3, and relay CRconstitute the relay driver portion of the circuitry. This portiontranslates the one second pulse into a one second relay closure. Therelay closure, in turn, applies voltage to lights L2 and L3, whichindicates a flaw has been detected.

Transformer TRl, bridge Bl, resistors R23, R24, R25, and P1, andcapacitors C8 and C9 convert AC voltage to adjustable DC voltage. MeterM2 indicates the current in the electromagnet. Light L1 indicates thecircuitry is energized. Diode D11 protects the windings of theelectromagnet from excessive transient voltage spikes. Fuse Fl protectsthe circuitry from current overloads.

We claim:

1. Apparatus for detecting elongate flaws in a magnetically energizedbody having a longitudinal extent, comprising:

first and second Hall devices for sensing magnetic flux fringing thesurface of said body, each of said devices including a pair of voltageline connection points and being arranged in a manner whereby a straightline extending between said voltage connection points of said firstdevice is generally parallel to a straight line extending between saidvoltage connection points of said second device and to said longitudinalextent;

said Hall devices being arranged with their respective sensing elementshaving planes which partially overlay and are parallel to each other,each of said planes being defined by the width and length of therespective element and being taken through the thickness of the element,and so that a transverse plane extending through the longitudinalextents of the straight lines, extending between the voltage connectionpoints of each device, deviates from a line normal to the overlayingplanes;

means for providing a flow of current through the conductor of saidfirst Hall device in a first direction;

means for providing a flow of current through the conductor of saidsecond Hall device in a direction opposite to said first direction;means for transmitting a first voltage signal indicative of the fringingmagnetic flux sensed by said first Hall device;

means for transmitting a second voltage signal indicative of fringingmagnetic flux sensed by said second Hall device; and

means for summarizing said first and second voltage signals andproducing an output indicative of the vector difference of said firstand second voltage signals.

2. The combination as described in claim 1, wherein:

said Hall devices are adaptedto generate voltages of substantially equalmagnitudes but of opposite polarity when positioned on opposing sides ofa flaw in the body being tested.

3. In magnetic flux detection apparatus for detecting fiaws inmagnetically energized material, said apparatus including a pair of Halldevices and characterized by:

said Hall devices being arranged with their respective sensing elementshaving planes which partially overlay and are parallel to each other,each of said planes being defined by the width and length of therespective element and being taken through the thickness of the element;

said Hall devices also being arranged so that a straight line, joiningthe points where the voltage lead lines are joined to the sensingelement of one device, is parallel to a corresponding straight line ofthe other device;

and so that a transverse plane extending through the longitudinalextents of said straight lines is other than normal to the overlayingplanes.

means for positioning the testmaterial so that said lines of saidelements are parallel to each other and to the longitudinal extent. ofthe flaw.

5. In a process of sensing magnetic flux for the detection of flaws inmagnetically energized material, the

4. In magnetic flux detection apparatus for detecting flaws, saidapparatus including magnetic energizer means having magnetic poles forimparting magnetic flux to material to be tested, and a pair of Halldevices for detecting flux fringing the surface of said material, saidapparatus being characterized by:

said Hall being arranged in partial overlaying relation in a directioncorresponding to a direction normal to the surface to be tested; andfurther comprising: means for positioning said devices so that astraight line, joining the points where the voltage lead lines arejoined to the sensing element of one device,is parallel to acorresponding line of the other device; and

step of:

positioning a pair of Hall generator devices in partially overlayingrelation in a direction corresponding to a direction away from thematerial being tested,

in a manner whereby a straight line, joining the points where thevoltage lead lines are joined to the sensing element of one device, isparallel to a corresponding line of the other device, and so that aplane extending through the longitudinal extents of said straight linesis other than normal to the planes of said devices, and said material isadvanced to position a longitudinally extending crack parallel to andbetween said straight lines.

6. Apparatus as described in claim 4 wherein:

said devices are positioned so that said straight line and saidcorresponding line are spaced apart generally one-sixteenth inch. in adirection corresponding to the direction of the field between

1. Apparatus for detecting elongate flaws in a magnetically energizedbody having a longitudinal extent, comprising: first and second Halldevices for sensing magnetic flux fringing the surface of said body,each of said devices including a pair of voltage line connection pointsand being arranged in a manner whereby a straight line extending betweensaid voltage connection points of said first device is generallyparallel to a straight line extending between said voltage connectionpoints of said second device and to said longitudinal extent; said Halldevices being arranged with their respective sensing elements havingplanes which partially overlay and are parallel to each other, each ofsaid planes being defined by the width and length of the respectiveelement and being taken through the thickness of the element, and sothat a transverse plane extending through the longitudinal extents ofthe straight lines, extending between the voltage connection points ofeach device, deviates from a line normal to the overlaying planes; meansfor providing a flow of current through the conductor of said first Halldevice in a first direction; means for providing a flow of currentthrough the conductor of said second Hall device in a direction oppositeto said first direction; means for transmitting a first voltage signalindicative of the fringing magnetic flux sensed by said first Halldevice; means for transmitting a second voltage signal indicative offringing magnetic flux sensed by said second Hall device; and means forsummarizing said first and second voltage signals and producing anoutput indicative of the vector difference of said first and secondvoltage signals.
 2. The combination as described in claim 1, wherein:said Hall devices are adapted to generate voltages of substantiallyequal magnitudes but of opposite polarity when positioned on opposingsides of a flaw in the body being tested.
 3. In magnetic flux detectionapparatus for detecting flaws in magnetically energized material, saidapparatus including a pair of Hall devices and characterized by: saidHall devices being arranged with their respective sensing elementshaving planes which partially overlay and are parallel to each other,each of said planes being defined by the widTh and length of therespective element and being taken through the thickness of the element;said Hall devices also being arranged so that a straight line, joiningthe points where the voltage lead lines are joined to the sensingelement of one device, is parallel to a corresponding straight line ofthe other device; and so that a transverse plane extending through thelongitudinal extents of said straight lines is other than normal to theoverlaying planes.
 4. In magnetic flux detection apparatus for detectingflaws, said apparatus including magnetic energizer means having magneticpoles for imparting magnetic flux to material to be tested, and a pairof Hall devices for detecting flux fringing the surface of saidmaterial, said apparatus being characterized by: said Hall beingarranged in partial overlaying relation in a direction corresponding toa direction normal to the surface to be tested; and further comprising:means for positioning said devices so that a straight line, joining thepoints where the voltage lead lines are joined to the sensing element ofone device, is parallel to a corresponding line of the other device; andmeans for positioning the test material so that said lines of saidelements are parallel to each other and to the longitudinal extent ofthe flaw.
 5. In a process of sensing magnetic flux for the detection offlaws in magnetically energized material, the step of: positioning apair of Hall generator devices in partially overlaying relation in adirection corresponding to a direction away from the material beingtested, in a manner whereby a straight line, joining the points wherethe voltage lead lines are joined to the sensing element of one device,is parallel to a corresponding line of the other device, and so that aplane extending through the longitudinal extents of said straight linesis other than normal to the planes of said devices, and said material isadvanced to position a longitudinally extending crack parallel to andbetween said straight lines.
 6. Apparatus as described in claim 4wherein: said devices are positioned so that said straight line and saidcorresponding line are spaced apart generally one-sixteenth inch in adirection corresponding to the direction of the field between saidpoles.